The Fused figure out how to sneak into the tower and corrupt the Sibling which is a spren that controls the tower and makes it so Fused can't use their powers in the tower. The corruption makes it so Fused can but Radiant fall asleep in the tower except for Kaladin (due to being of the 4th ideal?). Kaladin tries to stop them from corrupting the Sibling more by destroying the tower defenses as the Fused find them. The lead Fused convinces Navani to work with her to research how light works. Over the course of the book they figure out how to blend different type of light and even create an anti-light. This anti-light allows both sides to permanently kill Radiant's spren and Fused. In the end Navani uses it to kill the lead Fused and reverse the Sibling's corruption so they could retake the tower. Kaladin swears the fifth ideal which gives him Shardplate as well. Navani becomes a second Bondsmith when she bonds the Sibling to save the tower.
Flashbacks in this book are about Eshonai and Venli from the first contact of the listeners and humans all the way through the death of Eshonai. We see how Venli was corrupted by the Odium spren and got the listeners to try new forms of power. We also see how Venli eventually got access to an Honor spren and became one of the first listener Radiants in many years. At the end Rlain becomes the second and more listener's start becoming Radiants. While at the tower Venli works on gathering singers who don't want to be on either side and want to leave the fighting. At the end she finds out that some listeners were still alive, and her and a small group go to meet them.
Adolin and Shallan go to the Spren's dimension to try to convince more of them to bond humans so they can have more Radiant to fight on their side. Alodin ends up agreeing to be tried for the crimes of humanity in order to convince them. The judge is one of the Herald's hiding here trying to escape the war and get off the planet. Adolin almost loses but his dead-eye sword spren coming back to life to defend him convinces many of the spren that it wasn't humans fault. Spren and humans both agreed to end the bonds to prevent something? else from happening. Shallan finally stops fighting her alter egos and faces the fact that she killed her first bond as a child and Pattern is actually her second bond.
Dalinar became a Bondsmith at the end of the last book, which gives him greater than normal powers and allows him to open stormlight rifts. He is off fighting on another front of the war and is cut off from the tower during most of the book. In the end they crush the Fused in another country and defeat Taravangian's forces which betrayed them. At the end Taravangian is killed by Sazed, but at the same time Taravangian summons Odium and Sazed is able to kill Odium with his special sword. Taravangian becomes the new Odium without anyone realizing since his body was left behind.